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Abstract

Nonlinear optical phenomena are the illustrations of the nonlinear responses of polarizations of media to electric fields. Since the first demonstration of frequency doubling from a Ruby laser in 1961, nonlinear optics has been intensively investigated. It has variety of applications from chemical sensing to image restoration. An optical parametric oscillator (OPO) is a device whereby a pump frequency can be converted to two lower frequencies in a nonlinear medium. There are three key ingredients for an OPO, i.e. a pump laser beam, a nonlinear medium providing a gain for the oscillating parametric wave through the nonlinear polarization, and an optical feedback by use of a cavity. In an OPO based on a KTiOPO4 (KTP) and KTiOAsO4 (KTA) crystal composite consisting of periodically-switched plates, signal twins and idler twins are generated due to two quasi-phase-matching conditions, instead of a single signal and a single idler in a conventional OPO. The two signals or idlers within the twins have perpendicular polarizations. In addition, the frequency separation between the signal twins or idler twins, dictated by the length of each crystal plate, is insensitive to the pump frequency and crystal temperature. Therefore, the signal twins (idler twins) are highly coherent with each other. These three novel features make such a type of the OPO stand out for realizing certain applications such as image restoration and THz generation having unique advantages. Since the signal twins or idler twins have perpendicular polarizations, they can be used to restore blurred images, which is insensitive to polarization of an incoming beam. In contrast, such an application based on a conventional OPO is sensitive to the polarization of the input beam. In addition, the twins can be heterodyned in a photodiode when their frequency separation is around 100 GHz, which can be used to detect carbon dioxide with an increased sensitivity and to simultaneously clean up the images blurred by atmospheric turbulence. For the AFB-KTP composite, the twins can be used in chemical sensing with a high sensitivity. It is worth noting that the frequency difference of the twin beams can be designed to be cover the range of 100 GHz to a few THz. Our theoretical analysis reveals that THz waves generated by mixing signal twins in a nonlinear medium have much significantly reduced quantum noise, compared with that by mixing a signal and an idler from a conventional OPO. Raman spectroscopy is an important tool in studying properties of optical materials based on third-order nonlinear process. In order to analyze anti-Stokes signals, relatively high pump power is usually required. As a result, local heating effects make Raman peaks lose their frequency accuracy. Anti-Stokes signals can be enhanced in a waveguide structure of lithium niobate, such that relatively low pump power is needed for spectroscopic studies. Such an advantage may open up the possibility for developing a portable Raman spectroscopic device. In the waveguide, both the forward and the backward anti-Stokes spectra can be distinguished. By amplifying an anti-Stokes Raman-shifted output in a laser amplifier, the corresponding system can be used to extend the detectable wavelength range of a silicon avalanche photodiode. A coherent anti-Stokes Raman output can used for image restoration based on phase conjugation. In this dissertation, I will focus on our investigations of novel nonlinear optical phenomena and applications based on second-order and third-order nonlinear processes. First of all, I will present our result on the generation of signal twins from an OPO based on KTA crystal twins, then summarize our studies on the generation of the idler triplets by using an AFB-KTP composite and a bulk KTP crystal being placed in the same OPO cavity. I am also going to give an overview of our work on the generation of 2 micron from an AFB-KTP composite. Second, I will present our results on image restoration based on transmission and reflection configurations. The reflection geometry is more practical than the transmission geometry in a field application. Third, I will summarize our key results on the generation of terahertz (THz) waves by frequency-mixing idler twins in a nonlinear crystal. We have observed noise reduction and linewidth narrowing, resulting in the enhancement of the THz output power. Fourth, I are going to summarize our findings on the enhancements of forward and backward anti-Stokes signals generated by a lithium niobate waveguide. We have observed significant frequency shifts of certain Raman peaks.